The present invention relates to the discovery that pyrolysis of certain dihalogenated monosilanes or mixtures of dihalogenated monosilanes can result in the formation of a highly stable, highly abrasion resistant, photoconductive, dopable semiconductor film on a substrate.
Amorphous polymeric materials of silicon and hydrogen (hereafter referred to as a-SiH) have emerged as a new class of semiconductors in recent years. Such materials are described, for example, in D. Carlson, U.S. Pat. No. 4,064,521, issued on Dec. 20, 1976. The materials are generated as thin films from the decomposition of silane (SiH.sub.4) in electrical discharges or, less frequently, from the thermal decomposition of silane or higher hydrogen-containing silanes (e.g., Si.sub.2 H.sub.6, Si.sub.3 H.sub.8, etc.) as described in a PCT patent application by A. MacDiarmid and Z. Kiss published as International Publication No. WO 82/03069 dated Sept. 16, 1982.
When it is desirable to include additional elemental constituents in the amorphous films, co-reactants such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) are added to the starting materials. When fluorine is to be incorporated into an amorphous film, tetrafluorosilane (SiF.sub.4) is most commonly added to the reactant mixture. This is described for example in U.S. Pat. No. 4,217,374 granted to Ovshinsky and Izu on Aug. 12, 1980. Similarly, chlorine atoms can be incorporated into films via decomposition of mixtures of silicon tetrachloride (SiCl.sub.4) and silane or hydrogen (see, for example, V. Augelli and R. Murri, Mater. Chem. and Phys. (Switzerland) 9, 301-5 (1983)).
A disadvantage to the use of silane, disilanes or polysilanes (Si.sub.n H.sub.2n+2) as a starting material for generation of a-SiH relates to the high explosion hazard of silane/air mixtures. An operational consequence of this explosion hazard is the frequent use of inert carrier gases as diluents in these systems. Both discharge (plasma) and chemical vapor deposition (CVD) routes to a-SiH from silane also have disadvantageous features. Conventional discharge systems require relatively sophisticated and expensive equipment to remove the reaction by-products. Silane-based CVD systems, while simple, suffer from a tendency to undergo vapor nucleation unless the pressure is very low. This behavior restricts the range of permissible operating conditions for such thermal decompositions.
United Kingdom Patent No. 2,148,328, issued to M. Hirooka, et al., on May 30, 1985, teaches the decomposition of various silanes, including halosilanes (SiX.sub.4), cyclic polymeric halosilanes (SiX.sub.2).sub.n, where n is greater than or equal to 3, di- and polysilanes such as Si.sub.n HX.sub.2n+1 and Si.sub.n H.sub.2 X.sub.2n. These materials are decomposed via electric discharge, photolysis, high temperature, or catalytically and mixed with a requisite second stream consisting of a vapor phase material selected from the group consisting of H.sub.2, SiH.sub.4, SiH.sub.3 Br, or SiH.sub.3 I wherein the second stream has also been decomposed. The obvious disadvantage of such prior art, one which clearly distinguishes it from the instant invention, is the necessity of having two materials to decompose. Hirooka, et al. requires the second stream as the source of hydrogen to facilitate the reduction of the halosilane to the amorphous silicon. The instant invention, however, has sufficient hydrogen in the single component dihalomonosilane to produce the desired amorphous silicon film. Several of the molecular precursors claimed by Hirooka, et al., such as (SiH.sub.2).sub.3 or (SiX.sub.2).sub.3, where X=F or Cl have either not been reported as being prepared elsewhere in the literature or have proved exceedingly difficult to synthesize. Hirooka, et al., does not teach the synthesis of these elusive molecules.
United Kingdom Patent number 2,156,385, issued Oct. 9, 1985 to Tanaka et al., teaches the deposition of a silicon film on a heated substrate from a reactant fluorosilane gas in the plasma state. However, Tanaka et al. does not teach the use of chemical vapor deposition of dihalosilanes to form amorphous silicon-containing films.
Thus the instant invention is distinguished from prior art utilizing chemical vapor deposition of disilanes, polysilanes, and reduction systems requiring dual streams of starting materials. The instant invention is also distinguished from the prior art utilizing plasma deposition systems.